Recently with improvements in magnetic recording there has been a growing demand for a higher image quality and a higher audio quality. To meet such demand, it has been carried out to pulverize ferromagnetic particles and to increase the density of a magnetic recording medium. Also, an increased consumption of a magnetic recording medium may require that it is inexpensive. A magnetic recording medium comprising plural magnetic layers has been proposed to achieve the foregoing requirements, because an upper magnetic layer can provide characteristics for a high image quality, and a lower magnetic layer can provide characteristics for a high audio quality, thereby causing suitable ferromagnetic particles to be used. Further, the use of plural magnetic layers results in reduced costs because suitable materials can be used depending on each of the plural magnetic layers.
It is said that electromagnetic characteristics are effectively increased by improving the surface property of a magnetic layer.
Such electromagnetic characteristics of a magnetic recording medium should be evaluated on condition that the magnetic recording medium has an excellent running property of a predetermined level or higher. Therefore, in the magnetic recording medium, it is necessary that the magnetic layer and the backing layer should have a low friction coefficient in order to have an excellent running property.
The requisite of today's magnetic recording medium is that it must have excellent electromagnetic characteristics, excellent running durability, and excellent manufacturability.
In order to have excellent electromagnetic characteristics, it is necessary that the surface of the magnetic layer is made as smooth as possible. To this end, it is necessary that the surface of the support on which the magnetic layer is formed must be very smooth. Especially, in case of a magnetic recording tape comprising plural magnetic layers, the first magnetic layer influences the surface property of the tape itself. Therefore, the surface of the support which greatly influences the surface of the lower magnetic layer is important, and needs to be made smooth.
In order to have excellent running durability, it is necessary that the outer surface of the magnetic layer and the backing layer must have a low friction coefficient.
In general, the running property of the tape is determined depending upon the sliding property of the magnetic layer when the magnetic layer slides on a magnetic head or a cylinder, and depending upon the sliding property of the backing layer when the backing layer slides on a pole or a guide provided in a tape deck.
When the friction coefficient is high, problems such as slipping or sticking occur. In order to avoid such problems, it is known that lubricating agents may be added in or on the magnetic layer.
It is known that roughness of the back surface (i.e., the surface of the support opposite of the magnetic layers) of the support is preferably increased or a backing layer is provided thereon, because good friction coefficient is not obtained when the back surface of the support directly slides on a guide or a pole.
However, when the back surface of the support is made too rough, such roughness is transferred to the magnetic layer when the magnetic recording tape is wound, thereby deteriorating electromagnetic characteristics. Therefore, it is necessary that the roughness of the back surface of the support is not so large that it is transferred to the magnetic layer, and yet is enough to maintain good running property.
In order to have excellent manufacturability, in addition to plural magnetic layers, it is requested to have good running durability without forming a backing layer on the back surface of the support.
JP-A-63-175222 (The term "JP-A" as used herein means an "unexamined published Japanese patent application) proposes a support in which two kinds of spherical silicas are used as a filler, and dense protrusions are formed, thereby preventing the protrusions of the support from being deformed, keeping the friction coefficient constant, and preventing electromagnetic characteristics from being deteriorated.
However, when plural magnetic layers are formed on such a support in which two kinds of fillers each differing in size are added, the surface of the support is not made smooth. As a result, since the lower magnetic layer in the plural magnetic layers records audio signals and the ferromagnetic particles having a decreased specific surface area (S.sub.BET) and a large average particle size are used to obtain a lower cost and further the coating composition having a large surface tension is used, the roughness of the support is further amplified and thereby a coarse lower magnetic layer is obtained. Accordingly, when the upper magnetic layer is provided on the lower magnetic layer, the obtained magnetic layers become coarse as a whole. It follows that electromagnetic characteristics in the magnetic layers are not so much improved, and big fillers present on the back surface or in the backing layer break off or become dislodged, thereby lowering durability.
Consequently, the back surface of the conventional support cannot be used as it is without forming a backing layer.